1. Field of the Invention
An aspect of the present invention relates to an electrolyte for a lithium secondary battery and a lithium secondary battery including the electrolyte, and more particularly, to an electrolyte for a lithium secondary battery capable of sustaining good battery performance and suppressing a swelling phenomenon affecting the thickness of the battery caused from a gas generated in the battery and a lithium secondary battery including the electrolyte.
2. Description of the Related Art
Recently, in the rapid development of electronic, communication, and computer industries, small-sized light-weight high-performance portable electric apparatuses such as camcorders, mobile phones, and notebook PCs have been widely used. Therefore, demand for batteries having a light weight, a long life cycle, and high reliability have increased. Particularly, in comparison with conventional lead acid batteries, nickel cadmium batteries, nickel hydride batteries, and nickel zinc batteries, a rechargeable lithium secondary battery has three times energy density per unit weight and a high charging speed. Thus, the rechargeable lithium secondary battery has been widely researched and developed.
A positive electrode activation material of the lithium secondary battery is made of a lithium metal oxide, and a negative electrode activation material of the lithium secondary battery is made of a lithium metal, a lithium alloy, a crystalline carbon, an amorphous carbon, or a carbon complex. The lithium secondary batteries may be classified into a lithium ion battery, a lithium ion polymer battery, and a lithium polymer battery according to types of separators and electrolytes. In addition, the lithium secondary batteries may be classified into a cylinder type, a polygon type, and a coin type.
Since an average discharging voltage of the lithium secondary battery ranges from about 3.6 V to about 3.7 V, the lithium secondary battery can generate a higher power than alkali batteries, Ni—MH batteries, and Ni—Cd batteries. However, in order to generate such a high driving voltage, an electrolyte composite which is electro-chemically stable in a charging discharging range from 0 to 4.2 V is needed. For the reason, a mixture of a non-aqueous carbonate-based solvent such as ethylene carbonate, dimethyl carbonate, and diethyl carbonate is used for the electrolyte. However, in comparison with an aqueous electrolyte used for the Ni—MH battery or the Ni—Cd battery, the non-aqueous electrolyte having such a composition has problems in that, since ion conductivity is too low, battery characteristics at a high charging discharging speed deteriorate
At the initial charging period of the lithium secondary battery, lithium ions extracted from the lithium metal oxide of the positive electrode move to the carbon electrode, that is, the negative electrode to be intercalated to the carbon. Since the lithium ions are a highly reactive material, the lithium ions react with the carbon electrode to form Li2CO3, Li2O, or LiOH, so that a film is formed on a surface of the negative electrode. The film is called a solid electrolyte (solid electrolyte interface; SEI) film. The SEI film formed at the initial charging period has a function of preventing the lithium ions from reacting with the negative carbon electrode or other material during the charging and discharging periods. In addition, the SEI film serves as an ion tunnel for passing only the lithium ions. In general, the lithium ions may be subject to solvation, and thus, the lithium ions together with an organic solvent having a large molecule weight are co-intercalated on the carbon electrode, so that a structure of the negative electrode may be destructed. The ion tunnel has a function of preventing destruction of the structure of the negative electrode. When the SEI film is formed, the lithium ions do not react with the negative carbon electrode or other material as an undesired reaction, so that an amount of the lithium ions can be reversibly maintained. Namely, the carbon of the negative electrode reacts with the electrolyte at the initial charging period to form a passivation layer such as the SEI film on the surface of the negative electrode. Therefore, the electrolyte is not dissolved, but the charging and discharging can be stably performed (see J. Power Sources, 51 (1994), 79-104). As a result, after the initial charging and discharging periods, in the lithium secondary battery, an irreversible forming reaction of the passivation layer does not occur, and a stable life cycle can be maintained.
However, during the forming reaction of the SEI film, there is a problem of gas generation in the inner portion of the battery caused from the dissolvation of the carbonate-based organic solvent (see J. Power Sources, 72 (1998), 66-70). Examples of the gasses generated in the inner portion of the battery, include H2, CO, CO2, CH4, C2H6, C3H8, and C3H6 according to types of the non-aqueous organic solvent and the negative electrode activation material. Due to the gas generation in the inner portion of the battery, the thickness of the battery expands at the charging period. In addition, as time elapses after the charging period, electrochemical energy and thermal energy increase. Therefore, the passivation layer may be gradually destructed to expose the negative electrode. The exposed negative electrode continuously reacts with ambient electrolyte as a side reaction. At this time, the gas is continuously generated, so that an internal pressure of the battery increases. Due to the increase in the internal pressure, the polygon type lithium polymer battery swells in a specific direction, or a specific surface of the battery is deformed. Therefore, a problem with the adhesiveness between the electrode plates in the electrode assembly of the battery occurs, so that there is deterioration in performance and stability of the battery as well as difficulty in set-mounting the lithium secondary battery to portable electric apparatuses.
In order to solve the aforementioned problems, there is proposed a method of mounting a current breaker or a vent for emitting the electrolyte at the state that the internal pressure increases up to a predetermined level, thereby improving the stability of the secondary battery including the non-aqueous electrolyte. However, the method has a problem in that malfunctions or dangers may occur due to increase in the internal pressure. In addition, in order to suppress the increase in the internal pressure, there is proposed a method of injecting an additive into the electrolyte to change the SEI forming reaction. For example, in Japanese Patent Application Publication No. 9-73918, there is disclosed a method of adding a diphenyl picrylhydrazyl compound of less than 1%, thereby improving a high-temperature storage property of the battery. In addition, in Japanese Patent Application Publication No. 8-321312, there is disclosed a method of adding N-buthyl amine-based compound of from 1% to 20%, thereby improving life cycle and a long-time storage property of the battery. In addition, in Japanese Patent Application Publication No. 8-64238, there is disclosed a method of adding a calcium salt ranging from 3×10−4 mol to 3×10−2 mol, thereby improving a storage property of the battery. In addition, in Japanese Patent Application Publication No. 6-333596, there is disclosed a method of adding an azo-based compound for suppressing reaction between the electrolyte and the negative electrode, thereby improving a storage property of the battery. In addition, in Japanese Patent Application Publication No. 7-176323, there is disclosed a method of adding CO2 to the electrolyte. In addition, in Japanese Patent Application Publication No. 7-320779, there is disclosed a method of adding a sulfide-based compound to the electrolyte, thereby suppressing dissolvation of the electrolyte.
Conventionally, as described above, in order to improve the storage property and stability of the battery, a small amount of an organic or inorganic material is added to form and introduce a suitable film such as the SEI film on the surface of the negative electrode. However, the additive compound reacts with the carbon of the negative electrode at the initial charging discharging period according to unique electrochemical characteristics thereof, so that the compound may be dissolved or an unstable film may be formed. Therefore, the ion mobility in the battery deteriorates, and the gas generated in the inner portion of the battery, causes the internal pressure to increase. As a result, there are problems of deterioration in storage property, stability, life cycle, and capacitance of the battery.